In this paper we discuss results from a number of parametric analyses of the energy and cost influences of fenestration in a prototypical office building. The energy important parameters of fenestration, daylighting, and electric lighting were systematically varied in several climates using the DOE-2.1senergy simulation program to determine net annual results. Results are presented for two climate extremes; one heating-load dominated and the other cooling-load dominated. The increase or decrease of net annual energy consumption and peak electrical demand due to fenestration is demonstrated. Daylighting, is shown to be the single most important strategy to reduce energy use, but can be an energy and cost liability. Conditions under which these liabilities occur are discussed, and optimal design solutions for minimizing energy costs are suggested.